Layers and Slope-Streaks within Valleys along the Highland-Lowland Boundary (extra-detail mgnf - MULTISPECTRUM; credits: Lunexit)

Slope Streak formation is among the few known processes currently active on Mars. Since the Slope Streaks in this image appear to superpose (lie on top of) the surfaces between individual dunes, the Streaks most likely formed more recently and are younger than the dunes.

Many hypotheses have been proposed for the formation of Slope Streaks including dry avalanching, geochemical weathering, liquid stains or flows, and moisture wickering.
Recent observations from HiRISE images have revealed that the dark interior of Slope Streaks is lower in elevation than the surroundings, suggesting that material must have been removed in the formation of the streak.